IBM RT User's Setup Manual page 371

Options Installation
9-4
3. Install the rear cover.
• On the IBM 6150 system unit, arrange the cables so that you
can fit the rear cover in place.
Warning: Do not operate the IBM 6150 system unit for
an extended period of time without the rear cover in
place. It is recommended that 75 mm (3 inches)
minimum clearance be provided between the IBM 6150
system unit cover louvers and adjacent walls or
furniture to ensure adequate cooling.
AS 07A056